Word Origin & History
1560s, from M.Fr. distinguiss-, stem of distinguer, from L. distinguere "to separate between, separate by pricking," from dis- "apart" + -stinguere "to prick." The suffix -ish is due to the influence of many verbs in which it is the equivalent of O.Fr. -iss, ultimately from L. inchoative suffix -iscere
(this is also the case in extinguish, admonish, and astonish). Related: Distinguishing.